If this is a 9" long barrel and it is truly a 1/6 scale cannon then it should have a .50 Caliber bore. Am I correct? My 1/3 scale is a 1" bore.
If this is all correct I would use 2f and basically load it exactly like I would my Pedrosoli Queen Anne Pistol. It to is a 9" long X .50 caliber....,and if that load seems a bit light., at least nobody got hurt.
I would use Goex., however I have been known to use whatever I can in a cannon.,since it can be slow matched., and my Flintlocks cannot be.,and MUST have Goex.,therefore I tend to save the Goex for them.,I have even been know to use a synthetic in the pistols as well ., I would use a few grains of Goex down the barrel., chase that with Pyrodex., and a few grains Goex in the Flash-Pan. This way I could conserve on the "Damn near impossable to get Goex in the state of Washington.
Just load light and you can work up from there.